In recent years, lithium secondary batteries have been widely used as power supplies for driving small electronic devices and the like. Such lithium secondary batteries are mainly constituted of a positive electrode comprised of a lithium compound oxide, a negative electrode comprised of a carbon material or a lithium metal, and a nonaqueous electrolyte solution. As the nonaqueous electrolyte solution, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is used.
A lithium secondary battery using, for example, LiCoO2, LiMn2O4 or LiNiO2 as a positive electrode brings about a reduction of the battery performance, when a part of the solvent of the nonaqueous electrolyte solution locally undergoes an oxidative decomposition during the charging, because the decomposition products inhibit the desired electrochemical reaction of the battery. Such a reduction is considered to be attributed to an electrochemical oxidation of the solvent at the interface between the positive electrode material and the nonaqueous electrolyte solution.
Also, a lithium secondary battery using, for example, a highly crystallized carbon material, such as natural graphite or artificial graphite, as a negative electrode brings about a reduction of the battery performance, when the solvent of the nonaqueous electrolyte solution undergoes a reductive decomposition on the surface of the negative electrode during the charging. Even in the case of EC which is generally used as a solvent for the nonaqueous electrolyte solution, a part thereof undergoes a reductive decomposition during repeated charging and discharging.
As techniques for improving the battery characteristics of such lithium secondary batteries, there are known, for example, Patent Documents 1 to 6.
Patent Document 1 discloses a lithium secondary battery using a pentafluorobenzene compound, such as pentafluoroanisole, having an electron donating group. The coin battery shows a discharge capacity retentivity of about 80% after 200 cycles and, therefore, the cycle property is not satisfactory.
Patent Document 2 discloses that pentafluoroanisole is usable as an oxidation reduction reagent as means for protecting from chemical overcharging of nonaqueous electrolyte solution secondary battery but does not mention the cycling property thereof.
Patent Document 3 suggests a nonaqueous electrolyte solution containing 2-propynyl phenyl carbonate, while Patent Document 4 suggests a nonaqueous electrolyte solution containing 2-propynyl phenyl oxalate.
Patent Document 5 discloses in its Example 5 a nonaqueous electrolyte solution containing pentafluorophenyl methyl carbonate, while Patent Document 6 discloses a nonaqueous electrolyte solution containing pentafluorophenylmethane sulfonate as well as vinylene carbonate and/or 1,3-propane sultone.
In these nonaqueous electrolyte solutions, the battery characteristics are improved in a certain degree. However, there is an increasing demand for higher capacity and longer cycle life. Therefore, further improvement of battery characteristics is required.
Patent Document 1: U.S. Patent Application Publication No. 2002/110735
Patent Document 2: Japanese Unexamined Patent Publication No. H07-302614
Patent Document 3: Japanese Unexamined Patent Publication No. 2000-195545
Patent Document 4: Japanese Unexamined Patent Publication No. 2002-124297
Patent Document 5: International Publication No. 03/77351
Patent Document 6: International Publication No. 05/29631